1. Field of the Invention
The present invention relates to a self-refresh system for use in a dynamic random access memory (dynamic RAM or DRAM hereinafter) such as a field memory for serially reading-out and writing-in image data to form an image or picture of one field of a television system.
2. Description of the Prior Art
The field memory of the above type is different from a normal memory for storing normal data and is adapted to read and write the image in series, therefore, the field memory has such a property that each memory cell of the field memory is automatically refreshed at regular periods. The period for refreshing the memory cell array of the field memory is generally 16.6 milliseconds (ms) when using the NTSC standard television signal. On the other hand, the periods for refreshing the memory cell array of various standard dynamic RAMs such as 256 K/DRAM and 1 M/DRAM are respectively 4 ms and 8 ms, both of which are shorter than the period of 16.6 ms mentioned above; therefore, the memory cell array of such a dynamic RAM should be refreshed in any method. Generally, there is provided a refresh control unit in an external portion of a dynamic RAM so as to refresh the memory cell array thereof at a regular period.
In the conventional refresh system, there must be provided an external control unit for refreshing the memory cell array, resulting in that the design of the device employing a field memory has been complicated and the number of the parts thereof has been increased.